i run avast, zone alarm, firefox, trillian, netmeter, transwin, automatic wallpaper changer, refreshlock, my wireless card's wireless manager, and Steam, and I can play 3d games like TF2 with no issues due to slowdown. and i have 1 gig of ram.
try what i said, and if you don't see a change, then it isn't avast. and even if you do see a change, it merely means you need to change your avast settings and make them better.